well the engine shifts one way or the other depending if its in R or D. so in one gear it may push up one part of the dp one way or another to make the o2 hit the firewall or the dp to hit the flex. my bet is one of those two just gotta get under the car and find it.
my suggestion. ngk o2 sensor is shorter according to Matt. so might could try that. and you could also try new lower mounts if you haven't done those and you can try flipping your dogbones if you haven't already to hold things still.